The quickness a ball travels to the plate from the pitchers’ hand.
Per Alan Nathan: Assuming a release point at 55 ft. from home plate, here are the flight times to the front edge of home plate for different release speeds:
75 – 0.516
80 – 0.484
85 – 0.455
90 – 0.430
95 – 0.407
100 – 0.387